Deprecation of `new Buffer(string)`

[bjoluc]

Hi @doowb,

since new Buffer(string) is deprecated since Node v6 and emits a deprecation warning when run outside node_modules (such as when append-buffer is bundled in Node.js CLI apps), I'd like to ask for your opinion about replacing it with Buffer.from(string) when available. Fully replacing it would call for a v2 of this lib, likely taking a long time to be adopted (esp. given the large number of transitive dependents), so I'd check for Buffer.from to be defined instead. Happy to submit a PR if you agree.

Cheers!